Electrical machines, for example motors and generators, have electrical conductors, an electrical insulation and a laminated stator core. The insulation has the purpose of electrically insulating the conductors from one another, from the laminated stator core and from the environment. In the event of mechanical or thermal stress in the course of operation of the machine, cavities may form at the interfaces between the insulation and the conductor or between the insulation and the laminated stator core, in which sparks can form as a result of partial electrical discharges. The sparks can result in formation of “treeing” channels in the insulation. As a result of the “treeing” channels, there may be an electrical sparkover through the insulation. A barrier against the partial discharges is achieved through the use of mica in the insulation, this having a high partial discharge resistance. The mica is used in the form of mica particles in platelet form with a conventional particle size of several hundreds of micrometers up to several millimeters, the mica particles being processed to give a mica paper.
In the course of production of mica paper, the mica particles in platelet form are arranged in layers, such that the particles are arranged substantially parallel to one another, with overlapping of immediately superposed mica particles to form contact surfaces. Between the contact surfaces, as a result of van der Waals forces and hydrogen bonds, interactions form, which give the mica paper a high mechanical durability and hence a stable form.
In the production of the insulation, the mica paper is wound around the conductor to be insulated and impregnated with a resin. Subsequently the composite composed of the resin and the mica paper is hardened. In addition, the mica paper may be applied to a carrier fabric composed of glass or polyester, in which case the carrier fabric imparts additional stability to the mica paper. An adhesive bonds the carrier fabric and the mica paper to a mica tape. To avoid high temperatures in the conductor in the course of operation of the machine, heat has to be removed from the conductor to the environment. The thermal conductivity of the mica paper is only about 0.2 to 0.25 W/mK at room temperature, as a result of which the dissipation of heat from the electrical conductor is limited.
An improvement in the conduction of heat could be achieved either through a decrease in the thickness of the insulation or through improved thermal conductivity of the insulation. The use of aluminum oxide particles in platelet form rather than the mica particles in platelet form is known, aluminum oxide having a much higher thermal conductivity at about 25 to 40 W/mK than mica.
In the case of use of the aluminum oxide particles in platelet form, however, the disadvantage arises that the particle size is conventionally below 100 micrometers, as a result of which the resultant contact surfaces of adjacent aluminum oxide particles are so small that the interactions thereof to form a particle composite are only weak. This is accompanied by a low strength of this particle composite, as a result of which the production of the insulation paper from the aluminum oxide particles is difficult.
WO 2005/056696 A2 and DE 102 43 438 A1 describe pigments which have been surface-modified by colorants and have been coated by one or more layers of polymer. DE 1 590 341 A1 describes a mica insulating body having a binder composed of a silicone resin to which a silica-alumina ester has been added. DE 1 590 341 A1 describes a mica insulating body having a binder composed of a silicone resin to which a silica-alumina ester has been added. The thermally stable electrical insulating material according to EP 0 623 936 A1 comprises melamine resin fibers, polymer fibrils, and optionally synthetic resin powders and mineral fillers. U.S. Pat. No. 4,578,308 discloses a “prepreg sheet” comprising a mixture of alumina fibers as the main component, organic microfibers and a heat-curable resin.